Thank you again Grinch.
I have just spoken to a man called Gareth on a number that he left on my answer machine. He assures me that he will send me a refund. He said that I had been unsubscribed last week but I got another text yesterday. He said that MobyMagic have no control over the company sending the texts. How does that work? I asked if it was safe to top up the credit. He hesitated then said yes and said I would be refunded if I got anymore. Seems no-one has any control over this thing. I have bought a new sim now and will use that instead.
He couldn't give me details of when I was supposed to have subscribed. The whole thing seems a total scam to me. I will wait for the refund to appear before I believe it is resolved.